1. Field of the Invention
The present invention relates to the preparation of N-(dichlorophosphoryl)trichlorophosphazene [P.sub.2 NOCl.sub.5 ], and, more especially, to the preparation of N-(dichlorophosphoryl)trichlorophosphazene from phosphorus pentachloride and ammonium chloride.
2. Description of the Prior Art
The preparation of linear polychlorophosphazenes of the formula: EQU Cl.sub.2 (O)P[NPCl.sub.2 ].sub.n Cl
from N-(dichlorophosphoryl)trichlorophosphazene of the formula: ##STR1## is known to this art (described in European Patent Application published under number 0,026,685).
This phosphazene of formula I may be produced by decomposition of P.sub.3 NCl.sub.12 (structural formula [Cl.sub.3 P=NPCl.sub.3 ].sup.+ PCl.sub.6.sup.-) under the action of P.sub.2 O.sub.5, with the P.sub.3 NCL.sub.12 ) being prepared from phosphorus pentachloride and ammonium chloride (U.S. Pat. No. 3,231,327).
Such method requires the use of P.sub.2 O.sub.5, a compound which is awkward to handle on account, in particular, of the solid state thereof and its hydrophilicity, and the decomposition of P.sub.3 NCl.sub.12 is accompanied by the production of a large amount of POCl.sub.3 (7 moles per 3 moles of compound of formula I).
The proposal has been made (M. Becke-Goering and W. Lehr., Chem. Ber., 94 1591-4 (1961)) to decompose P.sub.3 NCl.sub.12 by means of SO.sub.2. According to these authors, P.sub.3 NCl.sub.12 is separated from the reaction medium and dissolved in liquid SO.sub.2. This technique requires the use of a large amount of SO.sub.2, and demands the prior separation of P.sub.3 NCl.sub.12, an operation which must of necessity be carried out while shielded from atmospheric moisture. J. Emsley and P. B. Udy J. Chem. Soc., (A) 3025-9 (1970), propose carrying out the synthesis of P.sub.3 NCl.sub.12 in symmetrical tetrachloroethane and injecting SO.sub.2 into the reaction medium. This technique gives rise, during the stage of preparation of the P.sub.3 NCl.sub.12, to the simultaneous formation of higher homologs (P.sub.3 N.sub.2 Cl.sub.9 and P.sub.4 N.sub.2 Cl.sub.14) as well as cyclic oligomers (N.dbd.PCl.sub.2).sub.n wherein n ranges from 3 to 7.
The treatment of a reaction medium of this type with SO.sub.2 causes a highly exothermic reaction which raises the temperature of the medium to the boiling point of the solvent. The reaction mixture becomes intense yellow in color and, after removal of the solvent, the crude product is orange-yellow. Experience has shown that distillation does not permit a colorless final product to be produced.